This invention relates to coating systems with modified crosslinkers. More specifically, it relates to radiation and thermal curing of coating systems containing modified crosslinkers, and to the use of modified amino resins as crosslinkers for coating systems to improve scratch and mar resistance.
Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Basecoat-clearcoat composite coatings are topcoats that offer exceptional gloss, depth of color, distinctness of image, or special metallic effects. The automotive industry has made extensive use of basecoat-clearcoat composite coatings for automotive body panels. Single layer topcoats and the clearcoats of color plus clear composite coatings usually require an extremely high degree of clarity and gloss to achieve the desired visual effect. Furthermore, they must maintain the clarity and gloss over long periods of time in the face of environmental challenges.
Clearcoat coating compositions used as the outermost automotive coating are subject to damage caused by numerous elements. These elements include environmental fall out, exposure to ultraviolet radiation from sunlight, exposure to high relative humidity at high temperature, and defects made by impacts of small, hard objects resulting in chipping. Topcoats and outer coatings in general can be formulated to reduce so called scratch and mar, on the one hand, and environmental etch on the other. Scratch and mar refers to damage from impact, rubbing, or abrasion that produces visible scratches or marring that sometimes can be rubbed out. xe2x80x9cEnvironmental etchxe2x80x9d is a term applied to a kind of exposure degradation that is characterized by spots or marks on or in the finish of the coating that often cannot be rubbed out.
In order to be commercially successful, a coating should provide as many favorable characteristics as possible. Accordingly, it is most preferable to produce a coating having an optimum mix of characteristics with regard to various forms of damage resistance. For example, it would be desirable to provide an increase in scratch and mar protection without lessening the environmental etch protection.
A number of coating systems have been optimized over the years to provide a favorable combination of these properties. However, because the systems represent a compromise, usually one property has been at least partially sacrificed to increase the other. For example, a harder clearcoat film may be more resistant to environmental etch, but may be more prone to damage by scratching. On the other hand, a softer clearcoat film may be more scratch resistant, but more susceptible to environmental etch.
In Gummerson, Journal of Coatings Technology, vol. 62, pages 43-49 (1990), acrylated melamines in UV curable coatings are disclosed. The acrylated melamines have both acrylic and alkoxy functionality. The melamines may be UV cured by a free radical mechanism. However, the melamines have no silicon-containing groups. Furthermore, there is no discussion of improved scratch and mar resistance in cured films containing the acrylated melamines.
In Strazik, U.S. Pat. No. 4,280,938, unsaturated condensates of an alkoxy-methylmelamine, an allyl alcohol and an acrylamide are combined with an unsaturated oil modified resin in air drying compositions. The condensates contain olefin functionality and alkoxymethyl groups but do not have any silicon-containing groups. Furthermore, radiation curing is not contemplated for the compositions containing the condensates.
In Deiner, U.S. Pat. No. 4,113,947, addition products are manufactured from the reaction of nitrogen-containing compounds that contain at least one ethylenically unsaturated radical (such as, for example, allylmelamine) with organopolysiloxanes which contain hydrogen atoms bonded to silicon. These addition products contain in their structure a silicon-containing group, but they have no remaining ethylenic unsaturation because the silicon-hydrogen bond of the organopolysiloxanes reacts with the ethylenic unsaturation of the nitrogen containing compounds. As such, the addition products are not suitable for use in radiation cured coatings.
An object of the invention is to provide compositions that can be added in small amounts to coating systems to increase scratch and mar resistance, while maintaining other desirable coating properties. A further object is to provide methods for making cured coatings having improved properties of scratch and mar resistance. Another object is to provide a method for radiation curing for improving the scratch and mar resistance of coatings.
These and other objects are achieved by the radiation and thermal curing of the compositions of the invention.
The invention provides a crosslinker for organic coatings having an amino resin core with substituents including an olefin functional group, a silicon-containing group, and at least one alkoxyalkyl or alkylol group. Coating compositions containing the crosslinker can be cured thermally and with radiation, especially ultraviolet radiation. The crosslinker can be included in a coating composition with one or more curable resins, applied onto a substrate, and cured to form the cured coating of the invention.
In a preferred embodiment, the crosslinker can be prepared by reacting a) alkoxyalkylated amino resin with b) an unsaturated compound having a functional group reactive toward alkoxyalkyl groups and c) a silicon-containing compound having a functional group reactive toward alkoxyalkyl groups. In one embodiment, a coating composition containing the crosslinker undergoes a stratification in the coating system to which it is applied, so that the crosslinker is present at a relatively greater concentration at the surface of the coating.